Aramid fibers are highly resistant to heat decomposition, have inherent flame resistance, and are frequently used in working wear for special environments where flame resistance is required. These and other inherent desirable properties of aramid fibers also create difficulties for fiber processing in other areas; specifically, aramids are difficult to dye.
A process for the continuous or semi-continuous dyeing of and simultaneously improving the flame-resistant properties of poly(m-phenyleneisophthalamide) fibers has been described by Cates et al in U.S. Pat. No. 4,759,770. The process includes the use of a fiber-swelling agent solution also containing one or more dyes and a flame retardant, the dye and the flame retardant being introduced into the fiber while in the swollen state. Suitable swelling agents described are dimethylsulfoxide (DMSO), dimethylacetamide (DMAC) and N-methylpyrrolidone (NMP).
Printing of aramid fabrics using a print paste composed of a polar solvent such as DMSO, DMAC or NMP, a dye, water and a print paste thickener is described in Hussamy U.S. Pat. No. 4,705,527; these print pastes may also include a flame retardant as in Hussamy U.S. Pat. No. 4,705,523. Aramid fabrics printed in a camouflage pattern have specific application for military use where personnel have the potential to be exposed to fire and flame. Fabrics made of highly crystalline aramid fibers, such as DuPont's Nomex.RTM. having high glass transition temperatures are difficult to print. The two Hussamy patents noted above describe procedures for obtaining printed aramid fabrics using polar solvents but the processes require some specialized equipment.
An exhaust process for dyeing or simultaneously dyeing and improving the flame resistance of aramid fibers using N-cyclohexyl-2-pyrrolidone (CHP) as a dye carrier together with a dyestuff in a single bath under conditions of elevated temperature and optionally elevated pressure is described in PCT/US88/04074 published as WO 89/06292 on Jul. 13, 1989 and issued as U.S. Pat. No. 4,898,596. The use of N-octyl-2-pyrrolidone (NOP) in dyeing aramid fibers is described in application Ser. No. 07/437,397 filed Nov. 16, 1989. Although residual NOP remaining on the fibers or fabric is usually removed from the dyed goods prior to further processing, we have found that residual NOP facilitates dyeing and flame retardant treating. This observation has suggested the application of NOP prior to dyeing and/or flame retardant treating aramid fabrics in general, regardless of prior processing if any, as a preparatory treatment.
Unlike the highly polar solvents such as DMSO, DMF and NMP which require about 60% concentration in aqueous solution to maintain their swelling of certain aramid fibers, NOP maintains its ability to permeate such fibers in concentrations of only about 0.5 to 1.0% in aqueous solutions. The ability to work at lower concentrations limits the damage this organic solvent causes to aramid fabrics as compared with other aprotic solvents.
We have discovered the advantages of a two-step process in which a dye diffusion promoting agent such as NOP is applied in an initial step prior to further processing such as dyeing or treating with a flame retardant or both. Initial treatment with a dye diffusion promoting agent such as NOP leaves residual NOP on the aramid fabric which may be sold to processors in this condition for subsequent dyeing and/or flame retardant treating. The separate application of the dye diffusion promoting agent prior to dyeing results in superior flame resistance and sometimes deeper dyeing than does the use of the dye diffusion promoting agent directly with the dye(s).
The preferred dye diffusion promoting agent NOP used in this invention is a somewhat volatile liquid and as such requires caution and care in commercial processing operations. It is convenient to use a solution of NOP alone as a separate bath prior to further processing as this allows recovery of the NOP in significant quantities and minimizes atmospheric escape of volatiles. This separate treatment also permits a higher degree of flexibility in further processing; dyebaths, especially aqueous dyebaths, flame retardants, various finishes, etc., may be used all independent of volatile NOP, further minimizing escape of volatiles and simplifying solution handling, clean-up and storage. The two-step process allows for the dyeing of fully or partially constructed garments by first treating the fabric with the dye diffusion promoting agent, an effective amount of which remains on the fabric. A garment is fully or partially constructed, then dyed to the appropriate shade.